PRIMITIVE TRACHEATES 401 



in the character of its breathing organs, which are air -tubes. 

 These are, however of very simple character, and open by very 

 numerous stigmata scattered over the surface of the body and 

 even present on the legs. The arrangement is not entirely 

 irregular, for some of these apertures are placed in a double 

 longitudinal row on the upper surface, while others are similarly 

 disposed on the under surface. We have, on the whole, what 

 may be considered a primitive or undifferentiated condition of 

 these organs, from which it is easy to imagine the derivation of 

 the more complex arrangements found in Myriapods, Arachnids, 

 and Insects. 



2. Annelid characters. — These will naturally be better appre- 

 ciated after the Annelids have been considered, and will only 

 be briefly enumerated. 



The body-wall, like that of an ordinary segmented worm, 

 consists of a thin skin covered by a delicate cuticle, and under- 

 lain by a muscular coat, consisting of an external layer of trans- 

 versely-running fibres and an internal layer of fibres having 

 a longitudinal direction. In regard to minute structure, the 

 muscle-fibres differ from those of Arthropods in being devoid of 

 transverse striations. 



The mouth of Peripatus leads into a muscular pharynx, like 

 that of many Annelids, but quite unlike what is to be found in 

 Arthropods. A much more striking feature is to be found in the 

 excretory organs. These consist of a series of tubes known as 

 nephridia, one of which opens at the base of almost every leg, 

 on its under surface. Such segmentally-arranged renal organs 

 are extremely characteristic of segmented worms. 



The central nervous system consists of a double brain- or 

 cerebral-ganglion above the pharynx, and two ventral cords con- 

 nected with these. A remarkable character is seen in the fact 

 that these cords are widely separated except at the extreme 

 posterior end of the body, where they unite together above the 

 gut. Very numerous slender transverse nerves connect these 

 two cords, much as the rungs of a ladder connect its sides, 

 though the "rungs" of the nervous system differ in being much 

 more narrow and numerous. The nerve cords are dilated at 

 regular intervals into ill-marked ganglia. This nervous system 

 agrees with that of various Annelids, and is also much like that 

 characteristic of the Pro to- Molluscs. 



VOL. I. 26 



